Cassini–Huygens is a joint NASA/ESA/ASI robotic spacecraft mission currently studying the planet Saturn and its moons. The spacecraft consists of two main elements: the NASA Cassini orbiter, named after the Italian-French astronomer Giovanni Domenico Cassini, and the ESA Huygens probe, named after the Dutch astronomer, mathematician and physicist Christiaan Huygens. It was launched on October 15, 1997 and entered into orbit around Saturn on July 1, 2004. On December 25, 2004 the Huygens probe separated from the orbiter at approximately 02:00 UTC; it reached Saturn's moon Titan on January 14, 2005 where it made an atmospheric descent to the surface and relayed scientific information. On April 18, 2008, NASA announced a two year extension of the mission. Cassini is the first spacecraft to orbit Saturn and the fourth to visit it.
Hundreds of scientists and engineers from 16 European countries and 33 of the United States make up the team responsible for designing, building, flying and collecting data from the Cassini orbiter and Huygens probe. The mission is managed by NASA’s Jet Propulsion Laboratory, where the orbiter was designed and assembled. Development of the Huygens Titan probe was managed by the European Space Research and Technology Centre, whose prime contractor for the probe is Alcatel in France. Equipment and instruments for the probe were supplied from many countries. The Italian Space Agency (ASI) provided Cassini's high-gain communication antenna, and a revolutionary compact and light-weight multimode radar (synthetic aperture radar, radar altimeter, radiometer).
Cassini has seven primary objectives:
The Cassini–Huygens spacecraft was launched on October 15, 1997 from Cape Canaveral Air Force Station's Launch Complex 40 using a US Air Force Titan IVB/Centaur launch vehicle. The launch vehicle was made up of a two-stage Titan IV booster rocket, two strap-on solid rocket motors, the Centaur upper stage, and a payload enclosure, or fairing. The complete Cassini flight system was composed of the launch vehicle and the spacecraft.
The total cost of the mission is about US$3.26 billion, including $1.4 billion for pre-launch development, $704 million for mission operations, $54 million for tracking and $422 million for the launch vehicle. The US contributed $2.6 billion, ESA $500 million and ASI $160 million.
The nominal end of the mission was in 2008 but an extension of the mission until 2010 has been approved. It is possible that funding will be granted for additional extensions. A list of Cassini–Huygens abbreviations is available.
Cassini–Huygens's origins date to 1982, when the European Science Foundation and the American National Academy of Sciences formed a working group to investigate future cooperative missions. Two European scientists suggested a paired Saturn Orbiter and Titan Probe as a possible joint mission. In 1983, NASA's Solar System Exploration Committee recommended the same Orbiter and Probe pair as a core NASA project. NASA and the European Space Agency (ESA) performed a joint study of the potential mission from 1984 to 1985. ESA continued with its own study in 1986, while American astronaut Sally Ride, in her influential 1987 report "NASA Leadership and America's Future in Space," also examined and approved of the Cassini mission.
While Ride's report described the Saturn orbiter and probe as a NASA solo mission, in 1988 the Associate Administrator for Space Science and Applications of NASA Len Fisk returned to the idea of a joint NASA and ESA mission. He wrote to his counterpart at the ESA, Roger Bonnet, strongly suggesting that the ESA choose the Cassini mission from the three candidates at hand and promising that NASA would commit to the mission as soon as ESA did.
At the time, NASA was becoming more sensitive to the strain that had developed between the American and European space programs as a result of European perceptions that NASA had not treated it like an equal during previous collaborations. NASA officials and advisers involved in promoting and planning Cassini–Huygens attempted to correct this trend by stressing their desire to evenly share any scientific and technology benefits resulting from the mission. In part, this newfound spirit of cooperation with Europe was driven by a sense of competition with the Soviet Union, which had begun to cooperate more closely with Europe as the ESA drew further away from NASA.
The collaboration not only improved relations between the two space programs but also helped Cassini–Huygens survive congressional budget cuts in the United States. Cassini–Huygens came under fire politically in both 1992 and 1994, but NASA successfully persuaded the U.S. Congress that it would be unwise to halt the project after the ESA had already poured funds into development because frustration on broken space exploration promises might spill over into other areas of foreign relations. The project proceeded politically smoothly after 1994, although, as noted below, citizens' groups concerned about its potential environmental impact attempted to derail it through protests and lawsuits until and past its 1997 launch.
The spacecraft was originally planned to be the second three-axis stabilized, RTG-powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. Cassini was being developed together with the Comet Rendezvous Asteroid Flyby (CRAF) spacecraft, but various budget cuts and rescopings of the project forced NASA to terminate CRAF development in order to save Cassini. As a result, the Cassini spacecraft became a more specialized design, canceling the implementation of the Mariner Mark II series.
The spacecraft, including the orbiter and the probe, is the largest and most complex interplanetary spacecraft built to date. The orbiter has a mass of 2,150 kg (4,739 lbs), the probe 350 kg (770 lbs). With the launch vehicle adapter and 3,132 kg (6,900 lbs) of propellants at launch, the spacecraft had a mass of about 5,600 kg (12,345 lbs). Only the two Phobos spacecraft sent to Mars by the Soviet Union were heavier. The Cassini spacecraft is more than 6.8 meters (22 ft) high and more than 4 meters (13 ft) wide. The complexity of the spacecraft is necessitated both by its trajectory (flight path) to Saturn, and by the ambitious program of scientific observations once the spacecraft reaches its destination. It functions with 1,630 interconnected electronic components, 22,000 wire connections, and over 14 kilometers (8.7 mi) of cabling.
Now that Cassini is orbiting Saturn, it is between 8.2 and 10.2 astronomical units from Earth. Because of this, it takes between 68 to 84 minutes for signals to travel from Earth to the spacecraft, and vice versa. Thus, ground controllers cannot give "real-time" instructions to the spacecraft, either for day-to-day operations or in cases of unexpected events. Even if they respond immediately after becoming aware of a problem, nearly three hours will have passed before the satellite receives a response.
Cassini's instrumentation consists of: a synthetic aperture RADAR mapper, a charge-coupled device imaging system, a visible/infrared mapping spectrometer, a composite infrared spectrometer, a cosmic dust analyzer, a radio and plasma wave experiment, a plasma spectrometer, an ultraviolet imaging spectrograph, a magnetospheric imaging instrument, a magnetometer and an ion/neutral mass spectrometer. Telemetry from the communications antenna and other special transmitters (an S-band transmitter and a dual-frequency Ka-band system) will also be used to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.
Plutonium power source and controversy
Because of Saturn's distance from the Sun, solar arrays were not feasible power sources for the spacecraft. To generate enough power, such arrays would have been too large and heavy. Instead, the Cassini orbiter is powered by three radioisotope thermoelectric generators (RTGs), which use heat from the natural decay of plutonium (in the form of plutonium dioxide) to generate direct current electricity. The RTGs have the same design as those on the Galileo and Ulysses spacecraft and are designed to have a long operational lifetime. At the end of the 11-year Cassini mission, they will still be able to produce at least 628 watts of power. One of Cassini's spare RTGs was used to power the New Horizons mission to Pluto and the Kuiper Belt.
The use of 32.8 kg (72 lbs) of plutonium—the most launched into space until then—attracted significant protest from environmental groups, physicists, and some former NASA staff. NASA made several statements intended to mean that the mission was acceptably safe: the chances of radioactive release during the first 3½ minutes after launch were 1 in 1,400; the chances of a release later in the rocket's climb into orbit were 1 in 476; the chances of the craft falling to Earth later were less than 1 in a million; a worst-case scenario would mean 120 humans could die from Cassini-caused cancer over 50 years. These figures were derided as wild guesses by commentators, including the theoretical physicist Professor Michio Kaku who suggested that 200,000 humans would die if the plutonium canisters survived reentry and crashed in a heavily populated area. His estimates were based on atmospheric dispersal of the plutonium over a metropolis however, and Cassini's launch trajectory did not bring it within suitable vicinity of any large metropolis. The design of the RTGs is such that they would be very unlikely to fracture even in the case of a catastrophic mission abort.
To gain momentum, Cassini's trajectory included several gravitational slingshot maneuvers: two passes of Venus, one of Earth, then one of Jupiter. The Earth fly-by was the final point when Cassini posed any danger to humans, and occurred successfully on August 18, 1999. Had it suffered a malfunction that caused it to impact, NASA's final environmental impact study estimated that in the worst case a significant fraction of the plutonium inside the RTGs would have dispersed into Earth's atmosphere, but the chances of that were nearly ten million to one. This worst case involved an acute angle of entry in which Cassini would gradually burn up and be vaporized in the upper atmosphere, a highly unlikely scenario. A small number of enviromentalists continued to protest after the maneuver.
The primary mission for Cassini ended on July 30, 2008, with a two-year mission extension already approved, and a second one possible. NASA is targeting decommissioning Cassini in 2012. Unlike the Galileo spacecraft, which was plunged into Jupiter to disintegrate in a fiery atmospheric entry, a similar approach for Cassini may impact a large object within the rings and make it uncontrollable. Instead, NASA is considering a high-altitude parking orbit and an impact on a smaller moon where RTG contamination will not be a problem.
The Huygens probe, supplied by the European Space Agency (ESA) and named after the 17th century Dutch astronomer Christiaan Huygens, scrutinized the clouds, atmosphere, and surface of Saturn's moon Titan in its descent on January 15, 2005. It was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface.
The probe system consisted of the probe itself which descended to Titan, and the probe support equipment (PSE) which remained attached to the orbiting spacecraft. The PSE includes electronics that track the probe, recover the data gathered during its descent, and process and deliver the data to the orbiter that transmits it to Earth. The data was transmitted by a radio link between Huygens and Cassini provided by Probe Data Relay Subsystem (PDRS). As the probe's mission could not be telecommanded from Earth because of the great distance, it was automatically managed by the Command Data Management Subsystem (CDMS). The PDRS and CDMS were provided by the Italian Space Agency (ASI).
Important events and discoveries
Venus and cruise to Jupiter
On August 18, 1999 at 03:28 UTC Cassini did a gravity-assist flyby of Earth. An hour and 20 minutes before closest approach, Cassini made the closest approach to the Moon at 377,000 km, and took a series of calibration images.
Cassini made its closest approach to Jupiter on December 30, 2000, and made many scientific measurements. About 26,000 images of Jupiter were taken during the months-long flyby. It produced the most detailed global color portrait of Jupiter yet (see image at right), in which the smallest visible features are approximately 60 km (40 miles) across.
The New Horizons mission to Pluto captured more recent images of Jupiter, with a closest approach on February 28, 2007.
A major finding of the flyby, announced on March 6, 2003, was of Jupiter's atmospheric circulation. Dark "belts" alternate with light "zones" in the atmosphere, and scientists had long considered the zones, with their pale clouds, to be areas of upwelling air, partly because many clouds on Earth form where air is rising. But analysis of Cassini imagery showed that individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts. According to Anthony Del Genio of NASA's Goddard Institute for Space Studies, "the belts must be the areas of net-rising atmospheric motion on Jupiter, [so] the net motion in the zones has to be sinking."
Other atmospheric observations included a swirling dark oval of high atmospheric-haze, about the size of the Great Red Spot, near Jupiter's north pole. Infrared imagery revealed aspects of circulation near the poles, with bands of globe-encircling winds, with adjacent bands moving in opposite directions.
The same announcement also discussed the nature of Jupiter's rings. Light scattering by particles in the rings showed the particles were irregularly shaped (rather than spherical) and likely originate as ejecta from micrometeorite impacts on Jupiter's moons, probably Metis and Adrastea.
Test of Einstein's theory of general relativity
On October 10, 2003, the Cassini science team announced the results of a test of Einstein's theory of general relativity, using radio signals from the Cassini probe. The researchers observed a frequency shift in the radio waves to and from the spacecraft, as those signals traveled close to the Sun. According to the theory of general relativity, a massive object like the Sun causes space-time to curve, and a beam of radio waves (or light) that passes by the Sun has to travel farther because of the curvature. The extra distance that the radio waves travel from Cassini past the Sun to Earth delays their arrival; the amount of the delay provides a sensitive test of the predictions of Einstein's theory. Although deviations from general relativity are expected in some cosmological models, none were found in this experiment. Previous tests by the Voyager probe were in agreement with the theoretical predictions with an accuracy of one part in one thousand. The Cassini experiment improved this to about 20 parts in a million, with the data still supporting Einstein's theory.
New moons of Saturn
Using images taken by Cassini, three new moons of Saturn were discovered in 2004. They are very small and were given the provisional names S/2004 S 1, S/2004 S 2 and S/2004 S 5 before being named Methone, Pallene and Polydeuces at the beginning of 2005.
On May 1, 2005, a new moon was discovered by Cassini in the Keeler gap. It was given the designation S/2005 S 1 before being named Daphnis. The only other known moon inside Saturn's ring system is Pan.
A press release on February 3, 2009 shows yet another new moon found by the Cassini Spacecraft. The moon is approximately 1/3 of a mile long in the G-ring of the ring system of Saturn.
On June 11, 2004, Cassini flew by the moon Phoebe. This was the first opportunity for close-up studies of this moon since the Voyager 2 flyby. It also was Cassini's only possible flyby for Phoebe due to the mechanics of the available orbits around Saturn.
First close up images were received on June 12, 2004, and mission scientists immediately realized that the surface of Phoebe looks different from asteroids visited by spacecraft. Parts of the heavily cratered surfaces look very bright in those pictures, and it is currently believed that a large amount of water ice exists under its immediate surface.
In an announcement on June 28, 2004 Cassini scientists described the measurement of the rotational period of Saturn. Since there are no fixed features on the surface that can be used to obtain this period, the repetition of radio emissions was used. These new data agree with the latest values measured from Earth, and constitute a puzzle to the scientists. It turns out that the radio rotational period has changed since it was first measured in 1980 by Voyager, and that it is now 6 minutes longer. This doesn't indicate a change in the overall spin of the planet, but is thought to be due to movement of the source of the radio emissions to a different latitude, at which the rotation rate is different.
On July 1, 2004, the spacecraft flew through the gap between the F and G rings and achieved orbit, after a seven year voyage.It is the first spacecraft to ever orbit Saturn.
The Saturn Orbital Insertion (SOI) maneuver performed by Cassini was complex, requiring the craft to orient its High-Gain Antenna away from Earth and along its flight path, to shield its instruments from particles in Saturn's rings. Once the craft crossed the ring plane, it had to rotate again to point its engine along its flight path, and then the engine fired to decelerate the craft and allow Saturn to capture it. Cassini was captured by Saturn's gravity at around 8:54 p.m. Pacific Daylight Time on June 30, 2004. During the maneuver Cassini passed within 20,000 km (13,000 miles) of Saturn's cloud tops.
Cassini had its first distant flyby of Saturn's largest moon, Titan, on July 2, 2004, only a day after orbit insertion, when it approached to within 339,000 kilometers (211,000 miles) of Titan and provided the best look at the moon's surface to date. Images taken through special filters (able to see through the moon's global haze) showed south polar clouds thought to be composed of methane and surface features with widely differing brightness. On October 27, 2004 the spacecraft executed the first of the 45 planned close flybys of Titan when it flew a mere 1,200 kilometers above the moon. Almost four gigabits of data were collected and transmitted to Earth, including the first radar images of the moon's haze-enshrouded surface. It revealed the surface of Titan (at least the area covered by radar) to be relatively level, with topography reaching no more than about 50 meters in altitude. The flyby provided a remarkable increase in imaging resolution over previous coverage. Images with up to 100 times higher resolution were taken and are typical of resolutions planned for subsequent Titan flybys.
Huygens encounter with Titan
Cassini released the Huygens probe on December 25, 2004, by means of a spring. It entered the atmosphere of Titan on January 14, 2005, and after a two-and-a-half-hour descent landed on solid ground with no liquids in view. Although Cassini successfully relayed 350 of the pictures that it received from Huygens of its descent and landing site, a software error failed to turn on one of the Cassini receivers and caused the loss of the other 350 pictures.
During the first two close flybys of the moon Enceladus in 2005, Cassini discovered a "deflection" in the local magnetic field that is characteristic for the existence of a thin but significant atmosphere. Other measurements obtained at that time point to ionized water vapor as being its main constituent. Cassini also observed water ice geysers erupting from the south pole of Enceladus, which gives more credibility to the idea that Enceladus is supplying the particles of Saturn's E ring. Mission scientists hypothesize that there may be pockets of liquid water near the surface of the moon that fuel the eruptions, making Enceladus one of the few bodies in our solar system to contain liquid water.
On March 12, 2008, Cassini made a close fly-by of Enceladus, getting within 50 km of the moon's surface.. The spacecraft passed through the plumes extending from its southern geysers, detecting water, carbon dioxide and various hydrocarbons with its mass spectrometer, while also mapping surface features that are at much higher temperature than their surroundings with the infrared spectrometer. Cassini was unable to collect data with its cosmic dust analyzer due to an unknown software malfunction.
Radio occultations of Saturn's rings
In May 2005, Cassini began a series of occultation experiments, to measure the size-distribution of particles in Saturn's rings, and measure the atmosphere of Saturn itself. For over 4 months, Cassini completed orbits designed for this purpose. During these experiments, Cassini flew behind the ring plane of Saturn, as seen from Earth, and transmitted radio waves through the particles. The radio signals were received on Earth, where the frequency, phase, and power of the signal was analyzed to help determine the structure of the rings.
Spoke phenomenon verified
In images captured September 5, 2005, Cassini finally detected spokes in Saturn's rings, hitherto seen only by famed visual observer Stephen James O'Meara in 1977 and later confirmed by the Voyager spacecraft in the early 1980s. The exact cause of the spokes is not yet understood; some models predicted spokes would not be visible again until 2007.
Lakes of Titan
Radar images obtained on July 21, 2006 appear to show lakes of liquid hydrocarbon (such as methane and ethane) in Titan's northern latitudes. This is the first discovery of currently-existing lakes anywhere besides Earth. The lakes range in size from about a kilometer to one which is one hundred kilometers across.
In November 2006, scientists discovered a storm at the south pole of Saturn with a distinct eyewall. This is characteristic of a hurricane on Earth and had never been seen on another planet before. Unlike a Terran hurricane, the storm appears to be stationary at the pole. The storm is 8,000 kilometers (5,000 mi) across, 70 kilometres (43 mi) high with winds blowing 560 kilometers per hour (350 mph).
On September 10, 2007, Cassini completed its flyby of the strange, two-toned, walnut-shaped moon, Iapetus. Images were taken from 1,000 miles (1,600 km) above the surface. As it was sending the images back to Earth, it was hit by a cosmic ray which forced it to temporarily enter safe mode. All of the data from the flyby was recovered.
On April 15, 2008, Cassini received funding for a two-year extended mission, currently underway. It consists of 60 more orbits of Saturn, and includes 21 more Titan flybys, seven of Enceladus, six of Mimas, eight of Tethys, and one targeted flyby each of Dione, Rhea, and Helene. The extended mission began on July 1, 2008, and has since been renamed the Cassini Equinox Mission.
The image above displays the initial gravity-assist trajectory of Cassini–Huygens. This is the process whereby an insignificant mass approaches a significant mass 'from behind' and 'steals' some of its orbital energy. The significant mass, usually a planet, loses a very small proportion of its orbital energy to the insignificant mass, in this case, the probe. However, due to the spacecraft's small mass, this energy transfer gives it a relatively large energy increase in proportion to its orbital energy, speeding its travel through space.
Cassini–Huygens performed two gravity assists at Venus, one at Earth and one at Jupiter.
The above simplified diagram shows, in two dimensions, the orbital motion of Cassini–Huygens on and after arrival at Saturn.Cassini's speed relative to the Sun. The various gravitational slingshots form visible peaks on the left, while the periodic variation on the right is caused by the spacecraft's orbit around Saturn. The data came from the JPL Horizons Ephemeris System in 2005. The speed above is instantaneous velocity in kilometers per second. The date/time is UTC in Spacecraft Event Time, which is from 1997-Oct-16 00:00:01 to 2008-Jul-06 23:59:59 UTC, two leap seconds during this period. Note also that the minimum velocity achieved during Saturnian orbit is more or less equal to Saturn's own orbital velocity, which is the ~5 km/s velocity which Cassini matched to enter orbit.
Published in July 2009.
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